FIG. 2 shows the blade surface Mach number at a blade cross-section on the tip side of a turbine rotor blade. The blade surface Mach number from the leading edge to trailing edge of a suction surface at the tip of a rotor blade is denoted by symbol Ms. The blade surface Mach number from the leading edge to trailing edge of a pressure surface is denoted by symbol Mp. As shown in FIG. 2, the blade surface Mach number of the suction surface indicates the maximum blade surface Mach number M_max at an intermediate portion between the leading edge and trailing edge of the blade and largely decreases from the intermediate portion to the trailing edge of the blade. A difference in the blade surface Mach number between the suction surface and the pressure surface produces a difference in pressure between the suction surface and the pressure surface, which will rotate the rotor blade.
However, if cooling air mixes in from a casing side, i.e., from the further outer circumferential side of the rotor blade, the cooling air interferes with the rotor blade. As shown in FIG. 3, the blade surface Mach number of the suction surface lowers as indicated by symbol M′_max, so that the pressure difference acting on the rotor blade decreases. This is because the interference of the cooling air with a mainstream fluid loses energy, which leads to no gas expansion. As a result, a total pressure loss increases at a blade cross-section of the tip of an airfoil.
In patent documents 1 and 2, the reason for the increasing total pressure loss lies in low-speed air that flows from the pressure surface toward the suction surface through between the tip and the casing. Thus, the technology is disclosed for sealing the flow of the low-speed air between the tip and the casing.
Patent document 3 proposes the following technology in addition to the technology for reinforcing the seal at the tip. An inflow angle with respect to the leading edge of a rotor blade is varied in a blade-height direction to reduce a blade-load on the tip. This reduces a difference in pressure between the suction pressure and the pressure surface, whereby a flow rate of low-speed air flowing from the pressure surface to the suction surface is reduced to achieve a reduction in loss.